In conventional steel production by means of electric arc furnaces, graphite or carbon electrodes employed for arc striking are consumed not only adjacent a lower end, where the arc is drawn or struck, but, also longitudinally along the periphery of the wearing section, generally as a result of the heat in the interior of the furnace. As a consequence, the electrode is consumed in such a way that a generally conical shape results at the lower end. The resulting reduction in diameter leads to a faster consumption of material at the electrode tip. In addition, due to strong vibration and a generally rough environment in the interior of the furnace, the tapering of the diameter of the wearing section causes ends of the electrode to break off precipitously. The resulting elevated consumption of wearing sections contributes to elevating costs incurred per ton of steel produced.
To avoid such disadvantages different types of electrodes have been developed, the metallic portion being coated with a protective layer or being water-cooled.
Water-cooled electrodes are known from, for example, DE-AS 2 430 817 and DE-AS 2 739 483. Such electrodes essentially comprise a water-cooled metallic upper section and a lower section of a consumable material, a so called wearing section generally made of graphite or carbon. The sections are often interconnected by means of a threaded nipple or the like. In the upper metal section or shaft, such electrodes include supply and return ducts for cooling water. The ducts may be grouped and interconnected or may comprise a central supply duct and a ring-shaped outer return duct. Such ducts generally approach a front or lower face of the metal shaft which borders on the wearing section, thus providing generally satisfactory cooling of this zone of interconnection. As a result, certain advantages are obtained, such as uniform temperatures in a peripheral direction with respect to the electrode, and an acceptable thermal balance in case of slight thermal stresses.
A further advantage is that the replacement of the consumed lower section by a new section does not involve any difficulties arising from material distortions attributable to excessively elevated temperatures in the region of the interconnection. Additionally, sealing between the electrode and an opening in the arc furnace cover through which the electrodes is inserted, does not generally promote major problems, due at least in part to the cross-section of the upper electrode section being relatively unchanged for lack of wear and tear and/or side oxidation.
Where such conventional water-cooled electrodes are employed, it is however, possible that the loss of the wearing section on account of external influences, such as traumatic scrap movement, vibration, uneven pressure, or the completely unnoticed consumption of the wearing section can lead to the development of an electric arc between the metal section or shaft and the broken tip of the wearing section and/or molten metal within the furnace, which can lead to the destruction of the water-cooled metal shaft. Apart from the fact that the destruction of the metal shaft is costly, as replacement by a spare shaft is required, generally inventoried, damage to the metallic section can pose great danger for the operating personnel and the furnace plant as a whole, since cooling water may pass from the destroyed metal section into the molten metal triggering a violent release of steam.
European patent application No. 0 012 573 (British Steel) describes a water-cooled electrode in which an inert gas is purged under pressure via appropriate ducting within the metal shaft or section, essentially in an axial direction, to the zone of interconnection between the metal shaft and the wearing section. If the consumable section is securely fastened and not defective, a relatively constant gas pressure will build up in the interconnection zone. A pressure drop may serve as a signal indicating that the active or wearing and metallic sections have broken contact, or that there must have been a major consumption or some other disruption to the connection between the two sections. One disadvantage to such an electrode is, however, that considerable, uncontrollable pressure losses may not produce clear signals, attributable generally to firstly, the contact areas of the two sections not being completely even and coplanar and, secondly, graphite being a porous material, which means that reliable sealing of the gas can be difficult. Thus it may be possible that the gas pressure registered within the interconnection zone can vary from electrode to electrode or even within the same electrode, depending on factors such as the type of consumable section being employed and its configuration.
Furthermore, on account of the fact that the gas monitoring duct is centrally located it is difficult to detect a partial fracture or one-sided consumption. Moreover, the signal obtained is an essentially a constantly rising curve, generally permitting only a delayed reaction to the signal.
Due to such a delay, the command for the interruption of the current supply and the removal of the electrode would also be delayed, opening the door to destruction of the metal shaft and thus a potential escape of coolant into the furnace.